wip

flock/README.md

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+# flock
+

flock/flock.go

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+// The flock package provides a file-system mediated locking mechanism on linux
+// using the `flock` system call.
+
+package flock
+
+import (
+	"errors"
+	"os"
+	"syscall"
+)
+
+// Lock gets an exclusive lock on the file at the given path. If the file
+// doesn't exist, it's created.
+func Lock(path string) (*os.File, error) {
+	return lock(path, syscall.LOCK_EX)
+}
+
+// TryLock will return a nil file if the file is already locked.
+func TryLock(path string) (*os.File, error) {
+	return lock(path, syscall.LOCK_EX|syscall.LOCK_NB)
+}
+
+func LockFile(f *os.File) error {
+	_, err := lockFile(f, syscall.LOCK_EX)
+	return err
+}
+
+// Returns true if the lock was successfully acquired.
+func TryLockFile(f *os.File) (bool, error) {
+	return lockFile(f, syscall.LOCK_EX|syscall.LOCK_NB)
+}
+
+func lockFile(f *os.File, flags int) (bool, error) {
+
+	if err := flock(int(f.Fd()), flags); err != nil {
+		if flags&syscall.LOCK_NB != 0 && errors.Is(err, syscall.EAGAIN) {
+			return false, nil
+		}
+		return false, err
+	}
+	return true, nil
+}
+
+func flock(fd int, how int) error {
+	_, _, e1 := syscall.Syscall(syscall.SYS_FLOCK, uintptr(fd), uintptr(how), 0)
+	if e1 != 0 {
+		return syscall.Errno(e1)
+	}
+	return nil
+}
+
+func lock(path string, flags int) (*os.File, error) {
+	perm := os.O_CREATE | os.O_RDWR
+	f, err := os.OpenFile(path, perm, 0600)
+	if err != nil {
+		return nil, err
+	}
+	ok, err := lockFile(f, flags)
+	if err != nil || !ok {
+		f.Close()
+		f = nil
+	}
+	return f, err
+}
+
+// Unlock releases the lock acquired via the Lock function.
+func Unlock(f *os.File) error {
+	return f.Close()
+}

flock/flock_test.go

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+package flock
+
+import (
+	"testing"
+	"time"
+)
+
+func Test_Lock_basic(t *testing.T) {
+	ch := make(chan int, 1)
+	f, err := Lock("/tmp/fsutil-test-lock")
+	if err != nil {
+		t.Fatal(err)
+	}
+	go func() {
+		time.Sleep(time.Second)
+		ch <- 10
+		Unlock(f)
+	}()
+
+	select {
+	case x := <-ch:
+		t.Fatal(x)
+	default:
+
+	}
+
+	f2, _ := Lock("/tmp/fsutil-test-lock")
+	defer Unlock(f2)
+	select {
+	case i := <-ch:
+		if i != 10 {
+			t.Fatal(i)
+		}
+	default:
+		t.Fatal("No value available.")
+	}
+
+}
+
+func Test_Lock_badPath(t *testing.T) {
+	_, err := Lock("./dne/file.lock")
+	if err == nil {
+		t.Fatal(err)
+	}
+}
+
+func TestTryLock(t *testing.T) {
+	lockPath := "/tmp/fsutil-test-lock"
+	f, err := TryLock(lockPath)
+	if err != nil {
+		t.Fatalf("%#v", err)
+		t.Fatal(err)
+	}
+
+	f2, err := TryLock(lockPath)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if f2 != nil {
+		t.Fatal(f2)
+	}
+
+	if err := Unlock(f); err != nil {
+		t.Fatal(err)
+	}
+}

flock/go.mod

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+module git.crumpington.com/lib/flock
+
+go 1.23.0